Method for supporting backward compatibility of mbms

ABSTRACT

A method for supporting backward compatibility of Multimedia Broadcast and Multicast Service (hereinafter referred to as MBMS) in a mobile communication system of Wideband Code Division Multiple Access (hereinafter referred to as WCDMA) is proposed in the present invention. The present invention provides the method for the mapping relationship between the MBMS service identifier and the RAB identifier and judging the capability of the RNC by the SGSN, and accordingly, it is made possible that MBMS service is compatible backward and equipments are compatible with different versions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

A method for supporting backward compatibility of Multimedia Broadcastand Multicast Service (hereinafter referred to as MBMS) in the mobilecommunication system of Wideband Code Division Multiple Access(hereinafter referred to as WCDMA) is proposed in the present invention.In order to guarantee the backward compatibility of MBMS and thecompatibility among the equipments with different versions, theinvention provides the method for obtaining the correspondingrelationship between MBMS service identifier and Radio Access Bearer(hereinafter referred to as RAB) identifier by a Service GPRS (GeneralPacket Radio Service) Support Node (hereinafter referred to as SGSN) anda User Equipment (hereinafter referred to as UE) when providing the MBMSservice via a Radio Network Controller (hereinafter referred to as RNC)that does not support the MBMS.

2. Description of the Related Art

Multimedia Broadcast and Multicast Service (hereinafter referred to asMBMS) is a new service under standardization by 3^(rd) GenerationPartnership Project (hereinafter referred to as 3GPP). The MBMS serviceis a unidirectional point-to-multipoint (p-t-m) service, whose mostremarkable characteristic is that it can make use of radio resources andnetwork resources efficiently.

FIG. 1 shows the MBMS system structure. The MBMS network structure,based on the core network of General Packet Radio Service (hereinafterreferred to as GPRS), has been added with new network units. Followingis the description on the MSMS system structure in FIG. 1.

Broadcast and Multicast Service Center 101 (hereinafter referred to asBM-SC) is a service control center of the MBMS system. Gateway GPRSSupporting Node 102 (hereinafter referred to as GGSN) and Service GPRSSupporting Node 103 (hereinafter referred to as SGSN) constitute thetransmission network of the MBMS service and provide route for datatransmission. UMTS Terrestrial Radio Access Network 104 (hereinafterreferred to as UTRAN) provides radio resources for the MBMS service overthe air-interface. User Equipment 105 (hereinafter referred to as UE) isthe terminal device for data receiving. Home Location Register 106(hereinafter referred to as HLR) saves the data related to users and canprovide services like user authentication. Uu107 is the radio interface,and Iu108 represents the interface between the access network and thecore network. Radio resources used by the MBMS service are not dedicatedfor one user, but for all users using this service.

For the interface Iu, all users that joins in the same MBMS serviceshare the same user interface. That is to say, one Radio NetworkController (hereinafter referred to as RNC) has only one MBMS datastream for the same MBMS service. In the RNC and the SGSN, therelationship of one-to-one correspondence lies between the userinterface and the MBMS service identifier and MBMS Bearer Context.

In the system of Rel99, Rel4 and Rel5, the Network Service Access PointIdentifier (hereinafter referred to as NSAPI) is one-to-onecorrespondent to the Radio Access Bearer (hereinafter referred to asRAB) identifier, and in the Packet Service domain (hereinafter referredto as PS domain), it is also one-to-one correspondent to the RadioBearer (hereinafter referred to as RB) identifier. When the UE initiatesthe process of activating the Packet Data Protocol (hereinafter referredto as PDP) context, it selects an unused NSAPI and transfers it to theSGSN. When the SGSN initiates the process of RAB allocating, it containsthe NSAPI into the information element of RAB identifier.

The process that an existing UE joins in the MBMS service is illustratedin FIG. 2.

201 the UE initiates the process of setting up the PDP Context. Havingbeen established successfully, the PDP Context is saved in the UE, theSGSN and the GGSN, and a signaling connection in PS domain isestablished between the UE and the GGSN. The intermediate devices forthe signaling connection are Radio Access Network (hereinafter referredto as RAN) and the SGSN.

202 the UE transmits an Internet Multicast Manage Protocol (hereinafterreferred to as IGMP) joining in message to the GGSN through thesignaling connection established in step 201. The message contains theparameter of Internet Protocol (hereinafter referred to as IP) multicastaddress, which can identify a certain MBMS multicast service or acertain multicast service in external data network.

203 the GGSN and the BM-SC perform the signaling interaction toauthenticate the UE.

204 the GGSN sends an “MBMS Notification Request” message to the SGSN,which contains parameters of UE identifier and IP multicast address.

205 after receiving the message in step 204, the SGSN sends a “MBMSContext Activation Request” message to the UE, which contains parametersof UE identifier and IP multicast address.

206 after receiving the message in step 205, the UE sends an “ActivateMBMS Context Request” message to the SGSN, which contains IP multicastaddress and Access Point Name (hereinafter referred to as APN).

207 the SGSN sends a “MBMS Notification Response” message to the GGSN,which contains the value of reason. The value of reason indicateswhether the MBMS context is activated successfully or not for the reasonof the SGSN or the UE. When receiving the response message of failure orthe activation overtime in the GGSN, the GGSN can return to the IPMulticast Access (which is defined in 3GPP TS29.061).

208 If necessary, encryption process is performed between the UE and theSGSN.

209 the SGSN sends a “MBMS Context Establishment Request” message to theGGSN, which contains IP multicast address and APN.

210 the GGSN performs the signaling interaction with the BM-SC toauthenticate the MBMS service and the UE after receiving the message instep 209.

211 If the GGSN has not established the bearer context for this MBMSservice yet, it sends a “Bearer Request” message to the BM-SC, whichcontains IP multicast address and the APN.

-   -   The BM-SC sends a “Bearer Response” message to the GGSN, which        contains bearer context information of this MBMS service. The        BM-SC adds the identifier of this GGSN into the downlink node        list of the BM-SC bearer context.    -   If this MBMS service has not yet been allocated with a Temporary        Mobile Group Identifier (hereinafter referred to as TMGI), the        BM-SC allocates a new one and transfers it to the GGSN and SGSN        by the message of “Bearer Response”, and to the UE by a message        of “Activate MBMS Context Accept” (215).

If the state of the bearer context is activating, the BM-SC initiatesthe session start process with the GGSN.

212 the GGSN generates the MBMS UE context and sends a “MBMS ContextGeneration Response” message to the SGSN.

213 If the SGSN has not yet established the bearer context for this MBMSservice, it sends the “Bearer Request” message to the GGSN, whichcontains the IP multicast address and the APN.

The BM-SC sends the “Bearer Response” message to the GGSN, whichcontains the MBMS bearer context information. The BM-SC adds theidentifier of this GGSN into the downlink node list of the BM-SC bearercontext.

If the state of the bearer context is activating, the BM-SC initiatesthe session start process with the GGSN.

214 if at least one RAB of PS domain has been established for this UE,the SGSN transfers the MBMS UE context information to the RAN.

215 the SGSN sends the “Activate MBMS Context Accept” message to the UE.The SGSN does not need to wait till the complete of step 212 to sendthis message.

In practical networks, it is possible that new equipments with MBMScapability coexist with the remnant ones that are in service currently.For instance, the SGSN that is capable of the MBMS can connect with theold version RNC. To guarantee the backward compatibility of the MBMSservice and the interworking among the equipments with differentversions, all old version equipments should provide the UE with the MBMSservice through the mode of Rel99.

But for the MBMS service, to activate the MBMS UE context when joiningin the MBMS service, the UE discriminates the MBMS contexts and notifiesthe SGSN with the MBMS service identifier instead of selecting the NSAPIfor the MBMS. Therefore, the SGSN has no NSAPI to utilize whenestablishing lu user interface for UE with the process of Rel99, and asthere is no correspondence relation between the NSAPI and the RABidentifier, the UE has no idea in discriminating the servicescorresponding to the data from the bearers. The SGSN can not replace theRAB identifier directly with the MBMS service identifier, for a RABidentifier is 8-bit long, while an MBMS service identifier, such asTMGI, is 32-bit long.

SUMMARY OF THE INVENTION

The object of present invention is to provide a method for supportingbackward compatibility of MBMS.

To realize above object, a method for supporting backward compatibilityof MBMS comprising steps of:

receiving a message of “Session Start” from a GGSN by a SGSN when theSGSN requires to send MBMS data including MBMS Session Start to a UE;

receiving a message of “Activate MBMS Context Request” from the UE bythe SGSN and sending a message of “Activate MBMS Context Accept” to theUE with the information exchange with the GGSN when the UE joins in thecase of the MBMS session is in operation;

receiving a message of “Service Request” or a message “Routing AreaUpdate” for the reason of “MBMS Service” from the UE by the SGSN whenthe UE move in with the MBMS session is in operation;

judging whether an interested RNC supports the MBMS or not by the SGSN;

sending the MBMS service to the interested UEs of the RNC in the mode ofRel99 by the SGSN if the RNC cannot support the MBMS; and

sending a message of “RAB Allocation Request” message to the RNC by theSGSN to establish lu user interface bearer for the UE, this messagecontains RAB identifier of the UE corresponding to the MBMS service.

With the method of providing the mapping relationship between the MBMSservice identifier and the RAB identifier and the method of judging thecapability of the RNC by the SGSN proposed in the present invention, itis made possible that MBMS service is compatible backward and equipmentsare compatible with different versions. The method proposed in thepresent invention is simple, reasonable and backward compatible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the view showing the logic network devices of the MBMSservice;

FIG. 2 illustrates the procedure that the existing UE joins in the MBMSservice;

FIG. 3 shows the procedure that the SGSN derives the mapping of the MBMSservice identifier and the RAB identifier;

FIG. 4 shows SGSN operation during the procedure that SGSN derives themapping of MBMS service identifier and the RAB identifier;

FIG. 5 shows the UE operation during the procedure that the SGSN derivesmapping of the MBMS service identifier and the RAB identifier;

FIG. 6 shows The procedure that the UE derives the mapping of MBMSservice identifier and the RAB identifier;

FIG. 7 shows MBMS session start signaling flow in case of UE derives themapping of MBMS service identifier and the RAB identifier;

FIG. 8 shows the UE operation during the procedure that UE derives themapping of MBMS service identifier and the RAB identifier;

FIG. 9 shows the SGSN operation during the procedure that the UE derivesthe mapping of MBMS service identifier and the RAB identifier; and

FIG. 10 shows the SGSN operation when MBMS session starts in the case ofUE derives the mapping of the MBMS service identifier and the RABidentifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to guarantee the backward compatibility of the MBMS and theinterworking among the equipments with different versions, the presentinvention provided the method for the SGSN in finding out whether theRNC support the MBMS or not and for the SGSN in specifying the mappingrelationship between the RAB identifier and the MBMS service identifier,comprising steps of allocating the mapping relationship between the MBMSservice identifier and the RAB identifier by the SGSN and allocating themapping relationship between the MBMS service identifier and the RABidentifier by the UE.

Above processes can be explained with figures. Flowcharts will be usedto describe the operating process of nodes (including the SGSN and theUE).

FIG. 3 shows the process that the SGSN allocates the mappingrelationship between the MBMS service identifier and the RAB identifier.When the MBMS data is to arrive (i.e., the SGSN receives the “MBMSSession Start Indication” message from the GGSN), or when the UE isjoining in the MBMS service (i.e., the SGSN receives the “Activate MBMSContext Request” message from the UE, and sends the “Activate MBMSContext Response” message to the UE after it exchanges information withthe GGSN, as shown from step 206 to 215 in FIG. 2), or when the UE ismoving into the MBMS service (i.e., the SGSN receives the messages of“Routing Area Update”, “MBMS Service Request” and so on from the UE withthe reason of “MBMS Service”), following process will be conducted: theSGSN judges whether the RNC in which the UE locates supports the MBMS ornot, provides the UE with the MBMS service in the mode of Rel99 for theRNC not supporting the MBMS in the MBMS service area, and obtains themapping relationship between the MBMS service identifier and the RABidentifier. In present invention, an example is taken to explain how theSGSN judges whether the RNC supports MBMS or not and how the SGSNallocates the mapping relationship between the MBMS service identifierand the RAB identifier when the MBMS session starts.

301 when transferring data, the SGSN receives the notification of datatransmission from the GGSN in advance. At this time, the SGSN searchesfor the relevant MBMS context to notify the RNC that is specified in thecontext of the data transmission. There are three approaches in servicefor notifying RNC of data transmission, i.e., the SGSN allocates therequest messages to the RNC by the message of MBMS notification, “MBMSSession Start” or “MBMS RAB Allocation Request”. It is assumed that theapproach of transferring the message of “MBMS Session Start” is adoptedin the present invention.

Three methods can be utilized to inform the SGSN whether the RNC inwhich the interested UE locates supports the MBMS or not. 301 a the SGSNsaves the information on whether the RNC supports MBMS or not, and theSGSN can learn about whether the relevant RNC supports the MBMS or notby searching for the information on its context. 301 b the SGSN sends a“MBMS Session Start Notification” message to the registered RNCs(indirectly or directly), and the RNCs that do not support the MBMS cannot understand the message, they send a “MBMS Session NotificationRefused” message to the SGSN. When the SGSN receives the “MBMS SessionNotification Refused” message and the refused reason is “someinformation units can not be understood” from the RNC, if the MBMSservice is in operation, the SGSN sends a “MBMS UE Connection Request”message to the RNC when the UE joins in or moves in. 301 c the SGSN setsa timer after it sends the “MBMS Session Start Notification” to the RNC,and if the SGSN does not receive any response message from the RNC atthe terminal time preset in the timer, it considers that the RNC doesnot support the MBMS. If the MBMS service is in operation, the SGSNsends the “MBMS UE Connection Request” message to the RNC when the UEjoins in or moves in.

The message of “MBMS Session Start” described above may containinformation on service identifier, RAB parameters, Quality of Service(hereinafter referred to as QoS) and so on. The detailed contents of theinformation do not have close relation with the present invention, sothey are omitted in the following.

302-308 show the operation flow of the RNC that does not support theMBMS.

302 by using one of the approaches, if the RNC in which a certaininterested UE locates does not support the MBMS and the UE stays in idlestate, then the SGSN sends the “Paging” message to the RNC.

303 the RNC begins to page the UE via the Paging Control Channel(hereinafter referred to as PCCH) after it receives the “Paging” messagefrom the SGSN.

304 If the UE stays in idle state, it initiates the process ofestablishing RRC connection (Radio Resource Control) while the lusignaling connection is established for the UE.

305 the SGSN specifies the mapping relationship between the MBMS serviceidentifier and the RAB identifier for the RNCs that do not support MBMS,and sends a “Return to Rel99 Mode” message to notify the UE. Thismessage contains the MBMS service identifier and the mappingrelationship between the MBMS service identifier and the RAB identifier.

The UE saves the MBMS service identifier and the mapping relationshipbetween the MBMS service identifier and the RAB identifier after itreceives the message of “Return Rel99 Mode” from the SGSN, and repliesthe SGSN with a message of “Return to Rel99 Mode Conformation”.

306 the SGSN initiates the process of establishing lu user interface forthe UE according to the QoS parameters corresponding to this MBMScontext. The SGSN sends the “RAB Allocation Request” message to the RNC.

307 After receiving the “RAB Allocation Request” message from the SGSN,the RNC allocates the relevant radio resources and sends a “RBEstablishment” message to the UE, and after receiving this message, theUE configures the corresponding resources and replies the RNC with amessage of “RB Establishment Completion”.

308 the RNC sends a “RAB Allocation Response” message to the SGSN.

309-316 show the operation flow of the RNC that support MBMS.

309 the RNC searches for this MBMS context after receiving the “MBMSSession Start” message from the SGSN (in this case, the RNC supports theMBMS). With the MBMS context, the RNC can learn about the number ofjoined UEs of each cell. The RNC determines whether to allocateresources in PTP or PTM mode according to the number of interested UBsof each cell. Here, the number of UEs is based on the pre-determinedcritical value of each cell. The RNC sends the message of “MBMSNotification” to every UE of the arriving of the data. If the number ofUEs is greater than the critical value, the RNC determines to allocateresource in PTM mode and the process skips the next steps of 210, 211,212 and 213 in this case. If the number of UEs is less than the criticalvalue, part of UEs are switched into the connection mode by the “MBMSNotification” message. The particular method of switching the UB intothe connection mode is beyond the present invention, so it is notdescribed in detail here.

310 If the UE stays in idle state and is required to switch into theconnection mode for the need of user counting, the UE initiates theprocess of the RRC connecting and the lu signaling connection isestablished for the UE.

311 If the UE has established the RRC connection and lu signalingconnection for the MBMS service, the SGSN sends the “MBMS UE ConnectionRequest” message to the RNC to make it initiate the process of MBMS UEconnecting. The “MBMS UE Connection Request” message contains theservice identifier list of MBMS that the UE joined in. This message canbe transferred via the dedicated lu signaling connection of the UE, thusthe RNC can learn about the identifier of the UE by searching for thecontext after receiving this message.

312 the RNC adds the information of this UE into the corresponding MBMScontexts respectively according to the MBMS service identifier list. Ifthe RNC has no service context corresponding to a certain MBMS service,it will create one for the MBMS service.

313 the RNC sends the “MBMS Connection Response” message to the SGSN.

314 the RNC counts the number of the interested UEs in the cellaccording to the MBMS context so as to determine whether to adopt thePTP transmission mode or the PTM one.

315 If the RNC has some UEs who joined in the MBMS service(corresponding to the described service when the MBMS session starts),it sends the Radio Access Network Application Part (hereinafter referredto as RANAP) message of “MBMS Session Start Response” to the SGSN.

316 the RNC establishes the radio bearer in the cell in which the userlocate. It determines whether to establish PTP radio bearer or PTM oneaccording to the number of users. Then, the RNC transfers the parametersof RB to the users of the cell. If the RNC determines to apply the PTMtransmission mode in step 209, the parameters of RB can be transferredto the UE through the message of “MBMS Notification” in this step.

317 the MBMS data is transferred to the UEs from the core network viathe established user interface.

FIG. 4 shows the operation flow of the SGSN during the SGSN allocatesthe mapping relationship between MBMS service identifier and the RABidentifier.

401 the SGSN receives the message from the GGSN. 402 if the message thatthe SGSN receives from the GGSN is a message of “Service StartIndication”, the process goes to 409; 403 the SGSN receives the messagefrom the UE. 404 if the message that the SGSN receives from the UE isthe message of “Service Request”, “Routing Area Update”, or the othermessages that the UE sends to the SGSN when the mobile UE in idle statehas not received the transmission mode or the RB parameters of thetarget cell. 405 the process goes to 409 if the reason for “ServiceRequest” or “Routing Area Update” is “MBMS Service”; 406 if the SGSNreceives the “Activate MBMS Context Request” message from the UE, 407the SGSN sends a message of “Activate MBMS Context Conformation” to theUE through the process that it exchanges information with the GGSN andthat GGSN exchanges information with the Broadcast and Multicast ServiceCenter (hereinafter referred to as BM-SC) to complete the process thatthe UE joins in the MBMS service, as shown from step 207 to step 215 inFIG. 2. 408 if the MBMS service is in operation, the process goes to409.

409 in the case that the UE joins in (406, 407) or moves in (403, 404)when the MBMS session starts or during the MBMS service, the SGSN needsto learn about whether the RNC in which the interested UE locatessupports the MBMS or not.

There are three methods that make the SGSN judge whether the RNC inwhich the UE locates supports the MBMS or not. The first is that: theSGSN saves the information on whether the RNC supports MBMS or not, andthe SGSN can learn about whether the RNC supports MBMS or not throughsearching for the information on its context. The second is that: theSGSN sends the “MBMS Session Start Notification” message to theregistered RNCs (indirectly or directly), and if it is replies with the“MBMS Session Notification Refused” message for “Some informationelements can not be understood”, then the SGSN can learn about whetherthe RNC supports the MBMS or not. If the UE joins in or moves in duringthe MBMS service, the SGSN sends the “MBMS UE Connection Request”message to the RNC. The third is that: a timer is set after the SGSNsends the “MBMS Session Start Notification” message to the RNC, and ifit is not responded from the RNC till the terminal of the timer, then itconsiders that the RNC does not support the MBMS. If the UE joins in ormoves in during the MBMS service, the SGSN sends the “MBMS UE ConnectionRequest” message to the RNC.

If the RNC in which the UE locates does not support the MBMS and the UElocates in the service area of the relevant MBMS, the SGSN provides theMBMS service to the UE in the mode of Rel99.

410 performing step 411 for the UEs in idle state. If the UE is in PMMconnection mode, the process goes to 413.

411 the SGSN sends the “Paging” message to the RNC that the UE joins in.

412 After receiving a “Paging Respond” message, the SGSN saves thesignaling connection identifier that RNC allocates to this UE andallocates the signaling connection identifiers in the SGSN.

413 the SGSN specifies the mapping relationship between the MBMS serviceidentifier and the RAB identifier for the RNCs that do not support theMBMS. 414 the SGSN notifies the UE of “Return to Rel99 Mode”. Thismessage contains the MBMS service identifier and the mappingrelationship between the MBMS identifier and the RAB identifier.

415 If the message that the SGSN receives from the UE is the “Return toRel99 Mode Conformation” message and 416 the SGSN initiates the processof establishing the user interface lu, then the SGSN sends the “RABAllocation Request” message to the UE and waits for the response.

417 If the SGSN is responded with the message of “RAB AllocationResponse”, then the RAB allocation has been completed successfully. 418the SGSN transfers the MBMS data to the RNC via the established userinterface.

FIG. 5 shows the operation flow of the UE during the process that theSGSN allocates the mapping relationship between MBMS identifier and theRAB identifier.

501 the UE receives message from the SGSN. 502 If the NAS message thatUE receives from the SGSN is the message “Return to Rel99 Mode”, 503then the UE saves the correspondence relationship between the MBMSservice identifier and the RAB identifier. 504 the UE sends the NASmessage of “Return to Rel99 Mode Conformation” to the SGSN.

The actions performed by the RNC when the SGSN allocates the mappingrelationship between the MBMS service identifier and the RAB identifierare just the same as that of the prior art, and detailed technicalcontents are omitted here.

FIG. 6 shows the process that the UE allocates the mapping relationshipbetween the MBMS service identifier and the RAB identifier. When joiningin the MBMS service, the UE selects an unused NSAPI and sends it to theSGSN. If the RNC of the cell where the UE locates in does not supportthe MBMS, then when SGSN initiates the process of RAB allocating, theNSAPI is included in the information element of the RAM identifier.

601 the UE initiates the process of establishing the PDP Context. Havingbeen established successfully, the PDP Context is saved in the UE, theSGSN and the GGSN, and the signaling connection in PS domain isestablished between the UE and the GGSN. The intermediate devices forthe signaling connection are the RAN and the SGSN.

602 the UE sends the message of “IGMP Joining” to the GGSN via thesignaling connection established in step 601. This message contains theparameter of IP Multicast Address, which can identify a certain MBMSservice or a certain multicast service of the external data network.

603 the GGSN and the BM-SC perform the signaling interaction toauthenticate the UE.

604 the GGSN sends the “MBMS Notification Request” message to the SGSN,which contains the parameters of the UE identifier and the IP multicastaddress.

605 after receiving the message in step 604, the SGSN sends the messageof “MBMS Context Activation Request” to the UE, which contains the UEidentifier and the IP multicast address.

606 after receiving the message in step 605, the UE selects the unusedNSAPI for this MBMS service. The message of “Activate MBMS ContextRequest” is sent to the SGSN, which contains the IP multicast address,the APN and the NSAPI.

607 after receiving the message in step 605, the SGSN saves the NSAPIthat the UE selects for this MBMS service. This NSAPI is utilized whenthe SGSN needs to provide this MBMS service to the UE with the Rel99mode. The application method of the NSAPI is in accordance with theexisting standards.

608 the SGSN sends the “MBMS Notification Response” message to the GGSN.This message contains the value of reason which indicates whether theMBMS context is activated successfully for the SGSN or the UE or not.Once receiving the response message of failure or the activationovertime in the GGSN, the GGSN can return the IP Multicast Access (whichis defined in 3GPP TS29.061).

609 If necessary, the encryption process is performed between the UE andthe SGSN.

610 the SGSN sends the message of “MBMS Context Establishment Request”to the GGSN, which contains the IP multicast address and the APN.

611 the GGSN performs the signaling interaction with the BM-SC toauthenticate the MBMS service and the UE after receiving the message instep 609.

612 If the GGSN has not established the bearer context for this MBMSservice yet, it sends the “Bearer Request” message to the BM-SC, whichcontains the IP multicast address and the APN.

The BM-SC sends the “Bearer Response” message to the GGSN, whichcontains the bearer context information of this MBMS service. The BM-SCadds the identifier of this GGSN into the downlink node list of theBM-SC bearer context.

If this MBMS service has not yet been allocated with the TMGI, the BM-SCallocates a new one and transfers it to the GGSN and the SGSN by themessage of “Bearer Response”, and to the UE by the message of “ActivateMBMS Context Accept” further (615).

If the state of the bearer context is activating, the BM-SC initiatesthe session start process with the GGSN.

613 the GGSN generates the MBMS UE context and transfers the “MBMSContext Generation Response” message to the SGSN.

614 If the SGSN has not yet established the bearer context for this MBMSservice, it sends the “Bearer Request” message to the GGSN. This messagecontains the IP multicast address and the APN.

The BM-SC sends the “Bearer Response” message to the GGSN, whichcontains the MBMS bearer context information. The BM-SC adds theidentifier of this GGSN into the downlink node list of the BM-SC bearercontext.

If the state of the bearer context is activating, the BM-SC initiatesthe session start process with the GGSN.

615 If at least one RAB of the PS domain has been established for thisUE, the SGSN transfers the MBMS UE context information to the RAN.

616 the SGSN sends the “Activate MBMS Context Accept” message to the UE.The SGSN does not need to wait till the complete of step 612 to sendthis message.

Corresponding to the process that the UE allocates the mappingrelationship between the MBMS service identifier and the RAB identifier,the UE selects the unused NSAPI and transfers it to the SGSN, and theSGSN saves this NSAPI. When the MBMS data is to arrive (i.e., the SGSNreceives the “MBMS Session Start Indication” message from the GGSN), orwhen the UE is joining in the MBMS service (i.e., the SGSN receives the“Activate MBMS Context Request” message from the UE, and sends the“Activate MBMS Context Response” message to the UE after it exchangesinformation with the GGSN, as shown form step 206 to 215 in FIG. 2), orwhen the UE is moving into the MBMS service (i.e., the SGSN receives themessages of “Routing Area Update”, “MBMS Service Request” one and so onwith the reason of “MBMS Service”), the SGSN provides the UE with theMBMS service in the mode of Rel99 on condition that the UE locates inthe MBMS service area but the RNC in which the UE locates does notsupport the MBMS. When the SGSN applies the message of “RAB AllocationRequest” to establish the user interface for this service, NSAPI isincluded in the information element of the RAB identifier. In thepresent invention, the process of the MBMS session start is taken as anexample to explain the relevant signaling flow, as shown in FIG. 7.

701 When transferring data, the SGSN receives the notification of datatransfer from the GGSN in advance. At this time, the SGSN searches forthe relevant MBMS context to notify the RNC that is specified in thecontext of the data transmission. There are three approaches in servicefor notifying the RNC of the data transmission: i.e., the SGSN allocatesthe request messages to the RNC by the message of MBMS notification,“MBMS Session Start” or “MBMS RAB Allocation Request”. It is assumedthat the approach of transferring the message of “MBMS Session Start” isadopted in the present invention.

Three methods can be utilized to inform the SGSN whether the RNC inwhich the interested UE locates supports the MBMS or not. 701 a the SGSNsaves the information on whether the RNC supports MBMS or not, and theSGSN can learn about whether the relevant RNC supports the MBMS or notby searching for the information on its context. 701 b the SGSN sendsthe “MBMS Session Start Notification” message to the registered RNCs(indirectly or directly), and the RNCs that do not support the MBMS cannot understand the message, they send the “MBMS Session NotificationRefused” message to the SGSN. When the SGSN receives the “MBMS SessionNotification Refused” message and the refused reason is “someinformation units can not be understood” from the RNC, if the MBMSservice is in operation, the SGSN sends the “MBMS UE Connection Request”message to the RNC when the UE joins in or moves in. 701 c the SGSN setsthe timer after it sends the “MBMS Session Start Notification” to theRNC, and if the SGSN does not receive any response message from the RNCat the terminal time preset in the timer, it considers that the RNC doesnot support the MBMS. If the MBMS service is in operation, the SGSNsends the “MBMS UE Connection Request” message to the RNC when the UEjoins in or moves in.

The message of “MBMS Session Start” described above may containinformation on service identifier, RAB parameters, Quality of Serviceand so on. The detailed contents of the information do not have closerelation with the present invention, so they are omitted in thefollowing.

702-707 show the operation flow of the RNC that does not support theMBMS.

702 by using one of the approaches, if the RNC in which a certaininterested UE locates does not support the MBMS and the UE stays in idlestate, then the SGSN sends the “Paging” message to the RNC.

703 the RNC begins to page the UE via the PCCH after receiving the“Paging” message from the SGSN.

704 If the UE stays in idle state, it initiates the process ofestablishing RRC connection while the lu signaling connection isestablished for the UE.

705 the SGSN initiates the process of establishing lu user interface forthe UE according to the QoS parameters corresponding to this MBMScontext. The SGSN sends the “RAB Allocation Request” message to the RNC.The NSAPI is included in the information element of RAB identifier.

706 After receiving the “RAB Allocation Request” message from the SGSN,the RNC allocates the relevant radio resources and sends the “RBEstablishment” message to the UE, and after receiving this message, theUE configures the corresponding resources and replies the RNC with themessage of “RB Establishment Completion”.

707 the RNC sends the “RAB Allocation Response” message to the SGSN.

708-714 show the operation flow of the RNC that support MBMS, which isin accordance with the prior art (as the corresponding step 309-315 inFIG. 3).

309 the RNC searches for this MBMS context after receiving the “MBMSSession Start” message from the SGSN (in this case, the RNC supports theMBMS). With the MBMS context, the RNC can learn about the number ofjoined UEs of each cell. The RNC determines whether to allocateresources in PTP or PTM mode according to the number of interested UEsof each cell. Here, the number of UEs is based on the pre-determinedcritical value of each cell. The RNC sends the message of “MBMSNotification” to every UE of the arriving of the data. If the number ofUEs is greater than the critical value, the RNC determines to allocateresource in PTM mode and the process skips the next steps of 709,710,711 and 712 in this case. If the number of UEs is less than the criticalvalue, part of UEs is switched into the connection mode by the “MBMSNotification” message. The particular method of switching the UE intothe connection mode is beyond the present invention, so it is notdescribed in detail here.

709 If the UE stays in idle state and is required to switch into theconnection mode for the need of user counting, the UE initiates theprocess of the RRC connecting and the lu signaling connection isestablished for the UE.

710 If the UE has established the RRC connection and lu signalingconnection for the MBMS service, the SGSN sends the “MBMS UE ConnectionRequest” message to the RNC to make it initiate the process of MBMS UEconnecting. The “MBMS UE Connection Request” message contains theservice identifier list of MBMS that the UE joined in. This message canbe transferred via the dedicated lu signaling connection of the UE, thusthe RNC can learn about the identifier of the UE by searching for thecontext after receiving this message.

711 the RNC adds the information of this UE into the corresponding MBMScontexts respectively according to the MBMS service identifier list. Ifthe RNC has no service context corresponding to a certain MBMS service,it will create one for the MBMS service.

712 The RNC sends the “MBMS Connection Response” message to the SGSN.

713 the RNC counts the number of the interested UEs in the cellaccording to the MBMS context so as to determine whether to adopt thePTP transmission mode or the PTM one. If the RNC has some UEs who joinedin the MBMS service (corresponding to the described service when theMBMS session starts), it sends the RANAP message of “MBMS Session StartResponse” to the SGSN.

714 the RNC establishes the radio bearer in the cell in which the userlocate. It determines whether to establish PTP radio bearer or PTM oneaccording to the number of users. Then, the RNC transfers the parametersof RB to the users of the cell. If the RNC determines to apply the PTMtransmission mode in step 209, the parameters of RB can be transferredto the UE through the message of 209 “MBMS Notification” in this step.

715 the MBMS data is transferred to the UEs from the core network viathe established user interface. If the MBMS data is transferred to theUE with the mode of Rel99, the UE can learn about which the receivedMBMS service is according to the NSAPI of corresponding bearer, i.e.,the UE can learn about the corresponding MBMS service identifier.

FIG. 8 shows the operation flow of the UE during the process that UEallocates the mapping relationship between the MBMS service identifierand the RAB identifier.

801 the UE receives message from the SGSN. 802 If the message that theUE receives from the SGSN is the message of “MBMS Context ActivationRequest”, 803 then the UE selects the unused NSAPI for this MBMSservice. 804 the UE sends the “Activate MBMS Context Request” message tothe SGSN, which contains the IP multicast address, the APN and theNSAPI.

FIG. 9 shows the operation flow of the SGSN during the process that theUE allocates the mapping relationship between the MBMS serviceidentifier and the RAB identifier.

901 the SGSN receives the message from the UE. 902 If the message thatSGSN receives from the UE is “Activate MBMS Context Request”, 903 thenthe SGSN saves the NSAPI included in the message. 904 the SGSN sends the“MBMS Notification Response” message to the GGSN. This message containsthe value of reason. The value of reason indicates whether the MBMScontext is activated successfully for the SGSN or the UE or not. 905 theSGSN carries out the process that other UEs join in the MBMS servicewith the attendance of the SGSN. FIG. 6 illustrates following processes:the safe encryption one, the one that the SGSN sends the “MBMS ContextGeneration Request” message to the GGSN and wait for response, the onethat the SGSN establishes the bearer context with the GGSN after itreceives the response message, the one that the SGSN provides MBMS UEcontext to the RAN after it receives the response message, and the onethat the SGSN sends the “Activate MBMS Context Accept” message to theUE. All the processes are based on the prior art, therefore, thedetailed technical contents are omitted here.

FIG. 10 shows the operation flow of the SGSN when the MBMS sessionstarts during the UE allocates the mapping relationship between the MBMSservice identifier and the RAB identifier.

1001 the SGSN receives the message from the GGSN.

1002 If the message that the SGSN receives from the GGSN is “MBMSSession Start”,

1003 then the SGSN judges whether the interested RNC supports the MBMSor not. The method for judgment is consistent with that illustrated inFIGS. 3 and 7. The process flow of the RNC that supports the MBMS isbased on the prior art, therefore, detailed technical contents areomitted here. If the interested RNC does not support the MBMS, 1004 theSGSN searches for the SGSN context for the NSAPI and QoS parameters ofMBMS service for the UE and generates the “RAB Allocation Request”message.

1005 the SGSN sends the “RAB Allocation Request” message to thecorresponding RNC and waits for response.

1006 If the SGSN receives the message of successful RAB allocation, 1007it transfers the MBMS data to the UE via the established user interface.

When the UE is joining in the MBMS service which is in operation (i.e.,the SGSN receives the “Activate MBMS Context Request” message from theUE, and sends the “Activate MBMS Context Response” message to the UEafter it exchanges information with the GGSN, as shown form step 206 to215 in FIG. 2), or when the UE is moving into the MBMS service which isin operation (i.e., the SGSN receives messages of the “Routing AreaUpdate”, the “MBMS Service Request” with the reason of “MBMS Service”and so on), the operation flow of the SGSN is in accordance with thatcarried out when the MBMS session starts on condition that the UElocates in the MBMS service area but the RNC in which the UE locatesdoes not support the MBMS.

1. A method for supporting backward compatibility of MBMS comprisingsteps of: receiving a message of “Session Start” from a GGSN by a SGSNwhen the SGSN requires to send a MBMS data including a MBMS SessionStart to a UE; receiving a message of “Activate MBMS Context Request”from the UE by the SGSN and sending a message of “Activate MBMS ContextAccept” to the UE with the information exchange with the GGSN when theUE joins in the case of the MBMS session is in operation; receiving amessage of “Service Request” or a message “Routing Area Update” for thereason of “MBMS Service” from the UE by the SGSN when the UE move inwith the MBMS session is in operation; judging whether an interested RNCsupports the MBMS or not by the SGSN; sending the MBMS service to theinterested UEs of the RNC in the mode of Rel99 by the SGSN if the RNCcannot support the MBMS; and sending a message of “RAB AllocationRequest” message to the RNC by the SGSN to establish lu user interfacebearer for the UE, this message contains RAB identifier of the UEcorresponding to the MBMS service.
 2. The method according to claim 1,wherein the method of allocating the RAB identifier for the MBMS serviceof the UE by the SGSN comprising steps of: allocating the mappingrelationship between the MBMS service identifier and the RAB identifierby the SGSN when the SGSN needs to send the MBMS data to the UE if theRNC does not support the MBMS; sending a message of “Return to Rel99Mode” to the UE and waiting for response by the SGSN if the RNC does notsupport MBMS; receiving the message of “Return to Rel99 Mode” by the UEfrom the SGSN; saving the correspondence relationship between the MBMSservice identifier and the RAB identifier contained in the message bythe UE; sending a message of “Return to Rel99 Mode Conformation” to theSGSN by the UE; receiving the message of “Return to Rel99 ModeConformation” from the UE by the SGSN; and sending a message of “RABAllocation Request” to the RNC by the SGSN.
 3. The method according toclaim 2, wherein the message of “Return to Rel99 Mode” containsinformation elements of MBMS service identifier and the mappingrelationship between the MBMS service identifier and the RAB identifier.4. The method according to claim 2, wherein the message of “Return toRel99 Mode Conformation” contains the information element of the MBMSservice identifier.
 5. The method of allocating a RAB (Radio AccessBearer) identifier for MBMS (Multimedia Broadcast and Multicast service)of the UE, comprising steps of: UE receiving a message of “MBMS ContextActivation Request” from the SGSN by the UE; allocating a NSAPI (NetworkService Access Point Identifier) for the MBMS service; sending themessage of “Activate MBMS Context Request” by the UE to the SGSN;receiving the message of “Activate MBMS Context Request” by the SGSNfrom the UE; and sending a message of “RAB Allocation Request” by theSGSN to the RNC if the RNC does not support MBMS when the SGSN needs totransfer the MBMS data to the UE, and this message contains RABidentifier corresponding to the NSAPI saved in the SGSN context.
 6. Themethod according to claim 5, wherein the message of “Activate MBMSContext Request” contains the NSAPI allocated for this MBMS service bythe UE.
 7. A method for establishing MBMS (Multimedia Broadcast andMulticast Service) bearer with a RNC(Radio Network Controller) which isincapable of MBMS, comprising steps of: receiving a message of “SessionStart” from a GGSN by a SGSN to send the message of “Session Start” to aUE; receiving a message of “Activate MBMS Context Request” from the UEby the SGSN and sending a response message for the request message;receiving a message of “Service Request” or a message of “Routing AreaUpdate” with a cause value regarding to “MBMS Service” from the UE bythe SGSN; sending a message of “RAB Allocation Request” message to theRNC by the SGSN to establish a bearer for the UE, if the RNC cannotsupport the MBMS; and sending a MBMS data to the UE which is located inthe RNC by the SGSN with the bearer.
 8. A method of claim 7, wherein themessage of “RAB Allocation Request” contains RAB identifiercorresponding to the MBMS service.